This invention relates generally to a method for measuring the moisture content of a bulk material, and specifically to such a method where the bulk material is soil in which turf or crops are cultivated.
The disclosed sensing method was designed to provide monitoring of the volumetric soil moisture content and report the same to a controller that activates irrigation valves in a manner so as to automatically irrigate the crops or turf. Many such systems of soil moisture monitors and controllers have been developed in the past with practically no commercial appearance in the irrigation control marketplace. Conductivity sensors, bulk dielectric constant sensors, time domain reflectometer (TDR) type sensors, and various oscillators using the dielectric constant of the water in the soil have been invented and patented with the hope for commercialization, yet acceptance has been marginal. This author has also experimented with and attempted to commercialize several of the above types with marginal results.
The major problem has been the impact of soil conductivity on the readings. Soil conductivity is a function of the ion content of the soil and of its temperature. Salts from irrigation water and/or fertilizer can build up in the soil and cause significant errors in moisture readings. Acidic or alkaline soils also have a detrimental effect on moisture readings taken by capacitive type or propagation type probes. At higher soil temperatures the conductivity increases and the errors become more pronounced.
Because of the uncertainty in the readings caused by this conductivity problem, many of the sensor offerings that have appeared in the marketplace have attempted to get by with xe2x80x98relativexe2x80x99 reading operation. This means that the sensor is installed in the soil (usually with a lot of water) and allowed to come to equilibrium with the surrounding soil for a week or so. A reference reading is then taken from the sensor at a point where the crop is judged to be xe2x80x98dryxe2x80x99 or in need of water. That reference reading becomes the threshold at which the controller is to apply water. It is assumed that whenever the sensor reading gets down to that reference level that the soil moisture is at the same xe2x80x98dryxe2x80x99 condition and is in need of replenishment. Unfortunately the readings from these xe2x80x98relativexe2x80x99 sensors do not remain in synchronism with the true or xe2x80x98absolutexe2x80x99 water content of the soil throughout the season. Ionic material and temperature have a major impact on that relationship and the crops tend to receive insufficient water as the seasonal temperature rise occurs and as salts build up in the soil. The solution to this problem is a sensor that measures the absolute amount of water in the soil under all practical conditions of salt content and temperature.
Based on experience with several types of sensors it was decided to pursue a propagation delay type sensor as the foundation upon which to build the solution. The author had his best experiences with a sensor similar to the one disclosed by Woodhead, et al in U.S. Pat. No. 5,148,125. This type of sensor relies on the fact that the propagation velocity of an electromagnetic wave in water is only 11% of what it is in air. A medium such as soil has a propagation velocity (v) according to:
v=c/{square root (k)}
wherein c is the velocity of light and k is the relative bulk dielectric constant of the medium. Water has a relative dielectric constant of about 78 at room temperature whereas the other components of soil rarely exceed a relative dielectric constant of 4. The relative bulk dielectric constant of a non-saturated soil-water mixture then ranges from around 4 to around 35 and the propagation velocity of a signal passing through soil ranges from about 0.5c to 0.17c respectively, being almost entirely dependent upon the water content.
Measuring the propagation time of a signal through soil is complicated by soil conductivity. Herein lies the source of the errors in present propagation-based, capacitance-based and conductivity-based sensors. For propagation-based sensors a transmission line of some sort is used to guide an electromagnetic wave through a specific length of soil. A wave is launched on the transmission line and received on the distant end or reflected back to the transmitter where its propagation time is measured. To simplify the propagation time measurement, the transmitted waveform is usually a pulse or step function with a very fast leading edge. Soil conductivity acts as a shunt loss mechanism to the transmission line. A mathematical analysis of such a model shows that the propagated waveform will have a reduction in amplitude and a dispersion of its leading and trailing edges. A transmitted pulse with 0.1 ns rise time could be received with several nanoseconds rise time and a greatly reduced amplitude. Deriving soil moisture data from such received waveforms has, in the past, been relegated to interpretation by trained personnel. Because of this complexity this method has not been commercialized as a means of automatically gathering soil moisture data.
This invention provides the means to launch a fast rising edge on a transmission line passing through a specific length of soil. The line folds back to a receiver mounted on the same circuit board as the transmitter. A precise timing and successive approximation amplitude-measuring scheme captures the timing and amplitude of the returning waveform with pico-second and milli-volt resolution, respectively. Front point-by-point waveform measurements, the slope along the returning waveform is examined. The point of the maximum slope is located and the propagation delay amplitude and slope at the maximum slope point are retained. A straight line of the same slope is then projected from the maximum slope point back to the baseline signal reference. The intersection of that line with the baseline reference represents the timing where the first returning energy from the transmitted edge reaches the receiver. This is the inflection point where the received waveform begins to turn upward. The timing of that intersection point is the true signal propagation time. As the slope and waveform amplitude change with soil conductivity that intersection point remains fixed. This intersection point propagation time, along with the dielectric constant of water (corrected for temperature) and the length of the transmission line are then used to calculate the soil moisture. Moisture readings so derived are virtually independent of the effects of ion content and temperature.